It is common practice while drilling a borehole to make measurements while drilling (MWD) or to log while drilling (LWD). The sensors used to make such measurements perform much better if they are in close contact with the formation of interest. Standoff, or gap, between the sensor and the formation reduces the measurement accuracy and the resultant test data are subject to correction for these gap errors. Thus, it is desired to place the sensors such that they are in substantial contact with the formation. To achieve such contact, or near contact, sensors are placed on the drill collar, and in some situations, on the outside of the drill collar. Traditionally, deep looking sensors used in LWD and MWD are mounted on the drill collar. In this approach, the sensors are displaced (typically a few inches) from the borehole wall. In many situations the depth of investigation of the tool is large (on the order of feet), and thus the effect of the gap between the borehole and the formation can be ignored or corrected.
Shallower looking devices, however, may be strongly affected by the borehole signal (the error signal caused by the gap), to the extent that, in extreme cases, the formation signal is overwhelmed by the borehole signal and rendered useless. Measurement techniques such as shallow resistivity, density, and neutron fall in this category. To avoid the borehole signal problem, the tendency is to place some sensors on parts of the drilling assembly that are in very close proximity to the formation.
U.S. Pat. No. 6,173,793 discloses a non-rotating sleeve for dealing with the standoff problem. The '793 patent suggests the idea of using a non-rotating sleeve on top of which the sensor is mounted. The sensor can be mounted on the surface of a fixed or an extendable pad that helps bring the sensor in contact with the borehole wall to eliminate, or at least reduce, the borehole signal. Designs are available to accommodate a variety of formation evaluation sensors, such as density, nuclear magnetic resonance (NMR), resistivity, acoustic, or electromagnetic.
U.S. Pat. No. 6,564,883 teaches the placement of sensors close to the formation. The '883 patent takes advantage of non-rotating sleeves that are used as one approach to achieve directional drilling. As with the '793 patent, the sensors are placed on a pad that is mounted on an extended rib, which in turn is attached to the non-rotating sleeve. The extended pad is actuated using hydraulic or electrical motors. Once actuated, the pad comes in contact with the surface of the borehole, allowing the sensors to make substantially borehole independent measurements.
U.S. Pat. No. 6,660,321 is directed to a resistivity device for imaging in wells drilled with oil-based mud. The '321 patent is directed to a pad, or an arm, that extends from a non-rotating sleeve. The '321 patent also suggests the possibility of using the stabilizer as a place for the sensor.
Published Patent Application US 2005/0001624 suggests a structure holding the sensor that protrudes from the drill string causing the sensors be closer to the borehole wall. The stabilizer is suggested as a place to position the sensors. The device described is for resistivity imaging of the borehole wall.
Published Patent Application US 2005/0006090 discloses the use of an extendable stabilizer with the sensor residing on the face of an extendable stabilizer. The stabilizer extends from the drill string to come in contact with the borehole wall. The extendable stabilizer helps bring the sensor closer to the borehole wall and reduces the borehole signal. The '090 application is for electrical imaging of the borehole wall.
All of the above-mentioned devices rely on non-rotating sleeves and suffer from at least three limitations. First, the non-rotating sleeve reduces the number of measurements. In the situation when the sleeve is completely stationary, the measurement from the sensor is limited to only one point on the circumference of the borehole wall, leading to limited azimuthal coverage. Even if the sleeve is slowly rotating, at higher rates of penetration the sensor may sample a helical path along the borehole wall and the resulting measurement will lack full coverage.
Second, the non-rotating sleeve is not used in all directional drilling approaches and may not be present in a drill string equipped with alternate directional drilling technologies that use rotating sleeves. The steering in such drilling strings uses hinged pads that contact the formation and it is this contact that changes the direction of the drill bit. Unlike the non-rotating sleeve approach, the hinged pads rotate at the same rotational speed as the entire drill string. At least one existing tool uses three such pads that can be activated/deactivated at three times the rate of drilling string rotation.
The third limitation is that the prior art structures are highly vulnerable to breakage when the tool is rotated. The inherent rugosity of wellbores and motion of the drill collar will cause the pad to be dragged against the wall of the wellbore, placing intolerable loads on the pad.
Patent Application 2005/0056421 shows the use of one or more pistons to carry one or more sensors, wherein each piston can extend to allow the sensor or piston face to contact the formation. In this approach the pistons are pressure balanced with downhole pressure such that the sensors or piston faces contact the borehole wall with minimum applied force and friction with the wall. This arrangement achieves the small standoff objective without producing excessive sensor or piston wear.
U.S. Pat. No. 5,242,020 show a drill string with a tangentially extendable arm deployable against the formation. Sensors are mounted within the arm. This arrangement is intended for situations in which the arm is to be used intermittently, not continuously while drilling. Though the description refers to “re-drilling”, this term means making a second pass subsequent to actually drilling the wellbore. The arm and sensors do not enlarge the hole during the subsequent pass as is the case when actually drilling or reaming a wellbore. Thus, the arm and sensors do not experience the extreme loads encountered in a true while-drilling or reaming situation.